This invention relates to optical sensors and relates more specifically to so-called spectral filtering optical sensors comprising a zone plate and associated input and output optical fibres having coterminous ends located at a predetermined distance from the zone plate and in which broadband light emerging from the end of the input fibre is arranged to be reflected by the zone plate before it impinges on the co-terminous end of the output optical fibre. Any variations in the effective optical path length between the zone plate and the ends of the optical fibres due to changes in a parameter (e.g. pressure, temperature, or linear dislacement) being sensed will produce changes in wavelength of the light impinging on the output fibre which can be detected and/or measured.
In known constructions of such spectral filtering optical sensors the zone plate is arranged to move linearly in response to changes in parameter in order to produce variations in optical path length between the zone plate and the co-terminous ends of the optical fibres. Alternatively, a movable plate mirror is interposed between the zone plate and the optical fibre ends which are maintained stationary.